The Detective's Shadow follows Kim Taek-rok, a veteran homicide detective nearing retirement who suddenly finds himself framed for a murder he did not commit. The true culprit, who identifies himself only as 'Friend', begins calling Taek-rok, taunting him with secrets from his past and forcing him to revisit old cases to clear his name. As the investigation deepens, Taek-rok must navigate a web of corruption within the police force while racing against time to uncover the identity of his tormentor. The series explores themes of redemption, the weight of past mistakes, and the moral ambiguity inherent in the pursuit of justice, as Taek-rok realizes that the shadow chasing him is closer than he ever imagined.